A vacuum-assisted countergravity casting process using a gas permeable, self-supporting mold sealingly received in a vacuum chamber is described in such patents as the Chandley et al. U.S. Pat. Nos. 4,340,108 and 4,606,396. That countergravity casting process involves providing a mold having a porous, gas permeable upper mold member (cope) and a lower mold member (drag) sealingly engaged together at a parting line, sealing the mouth of a vacuum housing to a surface of the mold such that a vacuum chamber formed in the housing confronts the gas permeable cope, immersing the bottom side of the drag in an underlying pool of melt, and evacuating the vacuum chamber to draw the melt upwardly through one or more ingate passages in the drag into one or more mold cavities formed between the cope and the drag.
Recent improvements in the vacuum-assisted countergravity casting process, represented by the Chandley U.S. Pat. No. 4,957,153; the Aubin et al. U.S. Pat. No. 4,971,131; and the Kubisch et al. U.S. Pat. No. 5,062,467 of common assignee herewith, have achieved substantial increases in the production and economies of the process. In these improved casting processes, one or more gas permeable molds, each typically comprising a pair of mated, relatively thin mold halves, are surrounded in a mass of particulate mold material (e.g., binderless foundry sand) held within the open bottom container by establishment of a suitable negative differential pressure between the inside and outside thereof. The particulate mass and the molds are held in the container such that lower melt ingate passages of the molds are exposed at the open bottom end of the container for immersion in an underlying melt pool. The negative differential pressure between the inside and the outside of the container is effective to draw the melt upwardly into the mold cavities formed by the molds in the particulate mass. After the melt has solidified in the molds and the container is moved to an unload station, the negative differential pressure is released to permit gravity-assisted discharge of the particulate mass, castings, and molds through the open bottom end of the container.
While the aforementioned improved countergravity casting processes are preferably practiced using unbonded (i.e., binderless) particulates held within the container by the negative differential pressure, the processes may also be practiced using weakly bonded particulates in a manner taught in the Plant U.S. Pat. No. 4,848,439 wherein the particulates are bonded in-situ in the container by passing a gas/vapor curing agent through binder-coated particulates after they are introduced in the container about the mold(s).
The aforementioned improved countergravity casting processes have exhibited capability to make thin walled castings of air melted alloys and also of vacuum melted alloys as described, for example, in U.S. Pat. No. 5,042,561.
These countergravity casting processes provided a major cost reduction in the production of many casting shapes as a result of reduced use of resin-bonded foundry sand needed for the molds and an increase in the number of castings made per casting cycle. However, in the production of more complex shaped castings having enlarged mold cavity regions isolated from the mold ingate passage entrances from high shrinkage alloys (such as stainless steels), higher cost per casting was experienced as a result of the need for an increased number of ingate passages and/or risers in the resin-bonded molds to supply adequate melt to the isolated, enlarged mold cavity regions. The increased number of ingate passages and/or risers resulted in additional resin-bonded mold sand usage, additional metal (melt) usage, and reduction in the number of castings made per casting cycle as a result of less available space in the vacuum housing, increasing the cost of making castings.
The additional risers needed were molded into the mold halves using appropriate resin-bonded cores. However, such cores can be used to form the riser in the mold only if the riser location is convenient to the mold parting line. Even then, the shape, size, and orientation of the riser are oftentimes restricted by molding process limitations.
It is an object of the invention to provide an improved casting apparatus and process of the type using a particulate mass disposed about one or more molds wherein the need for additional mold ingate passages and/or risers (and resultant additional usage of costly resin-bonded sand) to supply melt, especially of high shrinkage alloys, to isolated and/or enlarged mold cavity regions is eliminated by connecting a preformed riser-forming member to the mold so as to be disposed in the particulate mass and to communicate to a mold region needing supply of additional melt thereto, as necessary, during solidification of the melt in the mold to accommodate melt shrinkage.
It is another object of the invention to provide an improved casting apparatus and process of the type using a particulate mass disposed about one or more molds wherein a destructible, preformed riser-forming member is connected to the mold at one or more isolated and/or enlarged mold cavity regions and is destroyed and replaced by the melt during casting to form a riser of melt in the particulate mass for supplying additional melt to the regions, as necessary, during solidification to accommodate melt shrinkage.
It is still another object of the invention to provide an improved casting apparatus and process of the type using a particulate mass disposed about one or more molds wherein a destructible, preformed organic riser-forming member is connected to the mold at one or more isolated and/or enlarged mold cavity regions and is destroyed in a manner to selectively introduce carbon and/or supplemental heat to the melt forming the riser so as to increase its fluidity for better supply to the regions, as necessary, during solidification to accommodate melt shrinkage.